正则表达式，截断缩短文章并用省略号代替标点符号？

I'm trying to truncate a description field in PHP, and insert an ellipsis in a proper position, so the user can click and learn more. So I retrieve some characters from a field:

LEFT(html,340) as descr

Then set the variable...

$desc= $a['descr'];

Now comes the hard part... I want to find last punctuation marks in the sting, and then replace them with '...' because I believe it would be quite ugly to break a sentence in a random position. Then if the sentence is too long and without punctuation or if the outcome becomes too short, I would actually start replacing the first found space from the end.

$desc1=preg_replace('/([\.\-_,:].*)$/','...',$desc);

    $desc=strlen(strlen($desc1)>170&&340>strlen($desc1))?$descr1:
        preg_replace('/(\s.*)$/','...',substr($desc,0,300));

This is what I tried so far, the problem though is that it breaks the string at completely random position, but not from the position of the last punctuation mark. hmmm

Also it is powerless when couple of punctuation marks are found together. Finally I would want to replace dashes only if they're preceded by a space. But I can't seem to figure out how to do these things, I mean how to put it all together...

I'll be glad if you could advice something on any of the above points.

Aight, that's how I solved it

$desc1 = preg_replace('/[.,!:_]+[^.,!:_]*$/', '... ', $desc);
    $desc=(strlen($desc1)<170)?preg_replace('/[\s]+[^\s]*$/', '... ', $desc) : $desc1;

Here's my solution:

<?php

define("STR_LEN_THRESHOLD", 400); //Define the threshold in characters.

$punctuation = ".,!:_"; //Define punctuation marks
$string = "Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Vestibulum tortor quam, feugiat vitae, ultricies eget, tempor sit amet, ante. Donec eu libero sit amet quam egestas semper. Aenean ultricies mi vitae est. Mauris placerat eleifend leo. Quisque sit amet est et sapien ullamcorper pharetra. Vestibulum erat wisi, condimentum sed, commodo vitae, ornare sit amet, wisi. Aenean fermentum, elit eget tincidunt condimentum, eros ipsum rutrum orci, sagittis tempus lacus enim ac dui. Donec non enim in turpis pulvinar facilisis. Ut felis. Praesent dapibus, neque id cursus faucibus, tortor neque egestas augue, eu vulputate magna eros eu erat. Aliquam erat volutpat. Nam dui mi, tincidunt quis, accumsan porttitor, facilisis luctus, metus";
while (strlen($string) > STR_LEN_THRESHOLD) { //While string is too long
    $string = preg_replace("/[$punctuation][^$punctuation]+$/", "", $string); //Remove the last sentence
}
echo $string . "..."; //After enough was removed, add ...

This outputs:

Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Vestibulum tortor quam, feugiat vitae, ultricies eget, tempor sit amet, ante. Donec eu libero sit amet quam egestas semper. Aenean ultricies mi vitae est. Mauris placerat eleifend leo. Quisque sit amet est et sapien ullamcorper pharetra. Vestibulum erat wisi, condimentum sed, commodo vitae...